1. Field of the Invention
The present invention relates to a heat-sensitive recording sheet having excellent color-forming properties and stability of background brightness.
2. Prior Art
In general, heat-sensitive recording sheet is produced by applying on the paper surface the coating which is prepared by individually grinding and dispersing a colorless chromogenic dyestuff and a color-developing material, such as a phenolic substance, into fine particles, mixing the resultant dispersions with each other and then adding thereto binder, filler, sensitizer, slipping agent and other auxiliaries. When this sheet is heated, the coating undergoes instantaneously a chemical reaction which forms a color. In this case various bright colors can be advantageously formed depending upon selection of specific colorless chromogenic dyestuff.
As the heat-sensitive recording system is widely adopted and its applications are diversified, high speed recording and the improvement of image, that is, the improvement or resolution, have come to be required. Hence, the heat energy of the thermal heads in the recording equipment capable of such high density and such high speed is more minimized. Therefore, it is required that the heat-sensitive recording sheet have a higher color-forming sensitivity sufficient for producing clear chromogenic records with such small heat input from the thermal head.
On the other hand, there are many color-developing agents which were disclosed in the Japanese Patent Publication No. 14039/1970 and various literatures. Particularly, 4,4'-isopropylidendiphenol (bisphenol A) is now widely put into practical use, since it is stable, inexpensive and readily available. But 4,4'-isopropylidendiphenol has as its disadvantage a high heat color-forming temperature, so that recording-aptitude for a slight amount of heat energy is not adequate and the problems, such as sticking, etc., occurs easily.
The color-forming temperature of heat-sensitive recording sheets depends upon the melting of either one of color-forming materials composed of a chromogenic dyestuff and an acidic material, such as organic acids and phenols.
Where both color-forming materials have a higher melting point, a substance with a lower melting point is added thereto. In this case, if one of the color-forming materials is dissolved by melting this substance, it is possible to cause a color-forming reaction even at a lower temperature. Also, the Japanese Laid-Open Patent Applications Nos. 39139/1978, 26139/1978, 5636/1978, 11036/1978, etc. have proposed to add the following heat-meltable substances of a lower melting point as sensitizers or melting-point-depression-agents: various waxes, fatty acid amides, alkylated biphenyls, substituted biphenylalkanes, cumarin compounds, diphenylamines, etc.
In the method for addition of sensitizer, it is required to melt the sensitizer prior to the color-forming reaction, so that the thermal response for a slight amount of heat energy in short time-pulse cannot be obtained satisfactorily in the high speed dynamic recording. The following problems occur due to the liquidation of the meltable substance in the color-forming layer: adhesion of residues onto the thermal head, bleeding, smearing, ghost, etc. In this case, under storage conditions at high temperature and at high humidity, the coloring of the background occurs with the time elapsed and the contrast of the recording image may be often degraded. Also sufficient results are not always obtained by these methods.